There is a growing interest in using magnesium and magnesium alloys in a number of medical devices (such as sheet, rod, tube, nails, bone plate and endovascular stent, etc.), because of magnesium and magnesium alloy have low density, light weight, high strength, good biological compatibility and biodegradable properties etc. Therefore, magnesium alloys have great advantages and potential in the field of orthopedic instruments, interventional medical devices and dental care. Magnesium alloys with high specific strength, high specific stiffness, good machinability, and good damping ability are widely used in the field of automotive, aerospace, electronics and so on.
Magnesium alloys have a hexagonal close-packed structure and less slip system at low temperature, and are brittle, so there are significant limits in its application. Grain refinement is an effective method to improve the comprehensive performance of magnesium alloy. Not only the strength of magnesium alloy increases, but also can improve the plasticity by grain refinement. At present, there are several methods for grain refinement (such as powder metallurgy, rapid solidification, severe plastic deformation (SPD), etc.). The ultrafine-grained materials of larger size can be prepared by SPD, and SPD has no impurity or defect introduction like the other methods (such as powder metallurgy, spray deposition and rapid solidification, amorphous crystallization, etc.). Researchers focused more on Equal Channel Angular Pressing (ECAP) method which is one kind of SPD methods.
At present, ECAP has been used to process magnesium alloys to refine grains. In Application number CN201310355624.0 entitled “EXTRUSION DIE AND EXTRUSION METHOD OF RARE EARTH MAGNESIUM ALLOY”, Application number CN200910099591.1 entitled “METHODS AND BACK PRESSURE RECIPROCATING DIES TUNNEL OF MAGNESIUM ALLOY”, Application number CN200910071255.6 entitled “ROTATR EXTRUSION DIE AND METHOD OF MAGNESIUM ALLOY SQUARE BAR”, Application number CN200810233106.0 entitled “METHOD AND DIE OF CONTINUOUS ANGLE SHEAR MAGNESIUM ALLOY”, the ECAP dies were designed and used in pressing magnesium alloys.
Application number CN201310149560.9 entitled “METHOD FOR NANO-CRYSTALLINE MAGNESIUM ALLOY PREPARATION BY CONTINUOUS SEVERE PLASTIC DEFORMATION” discloses a method for manufacturing homogeneous magnesium alloys with the average grain size of below 100 nm by high pressure torsion after ECAP.
Application number CN201210516981.6 entitled “METHOD FOR HIGH YIELD OF ULTRAFINE CRYSTAL MAGNESIUM ALLOY SHEET PROCESSING” discloses a method for manufacturing a magnesium alloy sheet and a magnesium alloy wide plate by rolling after ECAP.
Application number KR20030060830 entitled “FORMING METHOD CAPABLE OF MINIMIZING GRAIN SIZE OF MAGNESIUM ALLOY BY IMPROVING MICROSTRUCTURE OF MAGNESIUM ALLOY THROUGH PLASTIC DEFORMATION OF MAGNESIUM ALLOY USING ECAP (EQUAL CHANNEL ANGULAR PROCESS” discloses a method for maximizing grain refinement of magnesium alloy when an ECAP is applied to magnesium alloy to increase ductility of magnesium, maintain a certain yield strength or more and expect to improve high temperature super plasticity according to grain refinement.
Application number KR20050024737 entitled “METHOD FOR MANUFACTURING HIGH STRENGTH/HIGH DUCTILITY MAGNESIUM ALLOY WITHOUT CHANGE OF MAGNESIUM ALLOY CONSTITUENTS BY CONTROLLING TEXTURE OF MAGNESIUMALLOY” discloses a method for manufacturing a magnesium alloy having strength that is far higher than that of an existing magnesium by controlling texture of the magnesium alloy by ECAP, and a method for manufacturing a magnesium alloy having strength similar to that of the existing magnesium and improved ductility by increasing ductility and minimizing yield strength reduced when using the ECAP.
Application number KR20050024735 entitled “METHOD FOR IMPROVING WORKABILITY OF MAGNESIUM AT ORDINARY TEMPERATURE BY DEVELOPING TEXTURE OF MAGNESIUM” discloses a magnesium alloy ECAP method for manufacturing magnesium alloy excellent in ductility by developing texture in magnesium.
The hydraulic equipments are used for ECAP in these above patents or applications, and their disadvantages are as follow: 1. The length of ultrafine-grained magnesium alloy prepared using hydraulic equipment is limited. The length of original preforms should be less than 100 mm due to the sizes of die and plunger. The final product does not exceed 80 mm because of the incomplete deformation areas; 2. In order to obtain the significant grain refinement of magnesium alloys, more than 8 passes of the ECAP have to be used. Accordingly, their production cost is high, and the production efficiency is low; 3. During ECAP process, once the pressing of one pass fails, the whole preform cannot continue to be used. And the incomplete deformation area accounts for about 20%, so the rejection rate is more than 1/4. 4. The prepared materials are preforms, and secondary processing is necessary.
In summary, the method with high production efficiency, low rejection rate, no upper size limit, and continuous production needs to be urgently exploited.